supplementary materials

Poly[[tetraaquadi-4-fumarato-2-oxalato-dierbium(III)] tetrahydrate]

The title compound, {[Er2(C4H2O4)2(C2O4)(H2O)4]·4H2O}n, was synthesized by the reaction of erbium nitrate hexahydrate with fumaric acid and oxalic acid under hydrothermal conditions. The Er3+ cation (site symmetry ..2) is eight-coordinated by six O atoms from four fumarate anions (site symmetry ..2) and one bidentate oxalate anion (site symmetry 222), and by two water molecules. The complex exhibits a three-dimensional structure consisting of oxalate pillared Er-fumarate layers with channels occupied by coordinating and lattice water molecules. The three-dimensional structure features by Owater-HO hydrogen bonds involving both the coordinating and lattice water molecules.

In recent years, lanthanide metal-organic compounds have been of great interest
due to their fascinating structures and potential applications in magnetism,
luminescence, catalysis, gas storage and separation. Multitopic carboxylates
have received considerable study due to their availability and potential for
allowing for the tailored design of such frameworks. As we know, fumaric acid
is a unique ligand with a relatively small, conjugated middle part and
versatile coordination modes. A large number of lanthanide metal complexes
containing fumarate ligands have been reported, see: Zhang et
al. (2006). And lanthanide-containing MOFs with two different
flexible
carboxylate ligands are less developed, see: Zhang et al.(2008);
Zhu
et al.(2007). In this paper, we report the synthesis and
structure of a
new metal-organic compound constructed from fumarate ligands coordinated to Er
atoms in the presence of oxalate ligands.

In the title compound I, Er1 is eight-coordinated with four O atoms from four
fumarate ligands (O2iii, O1iv, O2, O1v, (iii), 1.25 - x, 0.25 -
y, z; (iv) 1.5 - x, 0.5 - y, 1 - z; (v)
-0.25 + x, -0.25 + y, 1 - z), two O atoms from one
oxalate ligand (O3 and O3iii) and two water molecules (O4 and O4iii) (Fig.
1). The Er—O bond lengths are between 2.273 (3)–2.428 (3) Å. The Er atoms
are linked through bridging carboxyl groups of fumarate ligands to form
two-dimensional Er–fum layers in the ab plane (Fig. 2).
Along the c direction, the Er-fum layers are pillared by the
oxalic acid resulting in a three-dimensional structure. The framework contains
approximately 6.2 Å×11.1 Å rectangular channels along the [100]
direction. These channels are occupied by coordinated and free water molecules
(Fig. 3). The three-dimensional structure is stabilized by Owater—H···O
hydrogen bonds involving both the coordinated and free water molecules.

The H atoms attached to carbon were positioned geometrically and treated as
riding on their parent atoms, with C—H 0.93. The hydrogen atoms of the
water molecules were located in difference maps and refined by using the
'DFIX' command with O—H = 0.85 (2)Å with Uiso(H) =
1.5Uiso(O).

Geometry. All e.s.d.'s (except the e.s.d. in the dihedral angle between two l.s. planes)
are estimated using the full covariance matrix. The cell e.s.d.'s are taken
into account individually in the estimation of e.s.d.'s in distances, angles
and torsion angles; correlations between e.s.d.'s in cell parameters are only
used when they are defined by crystal symmetry. An approximate (isotropic)
treatment of cell e.s.d.'s is used for estimating e.s.d.'s involving l.s.
planes.

Refinement. Refinement of F2 against ALL reflections. The weighted R-factor
wR and goodness of fit S are based on F2, conventional
R-factors R are based on F, with F set to zero for
negative F2. The threshold expression of F2 >
σ(F2) is used only for calculating R-factors(gt) etc.
and is not relevant to the choice of reflections for refinement.
R-factors based on F2 are statistically about twice as large
as those based on F, and R- factors based on ALL data will be
even larger.